1. Field of the Invention
The present invention relates to a method and a device for measuring the resistivity anisotropy of rocks exhibiting layerings such as laminations. This layering is due to the existence of clay layers or of compacted sand beds of different grain sizes. When the formation is 100% water saturated, the resistivity contrast is low (factor 3 for example). But in oil zones, this contrast is very high because of a very different water saturation.
Measurement of the resistivity index of core samples exhibiting such layerings is necessary to obtain a precise estimation of the water saturation from log data obtained for example by means of the measuring while drilling (MWD technique).
Knowledge of the resistivity anisotropy in two preferred directions is useful to determine the water saturation of the porous medium present between the laminations. In fact, the vertical resolution of well logging tools is often insufficient to detect the resistivity fluctuations resulting from the accumulation of the various layerings according to the depth. Furthermore, the laminations contain few hydrocarbons and can be of very low permeability because they often essentially consist of clay. Thus, it is well-known that measurement of the mean resistivity in a direction does not allow determination of the water saturation in the layers that can produce hydrocarbons.
2. Description of the Prior Art
EP-701,128 (U.S. Pat. No. 5,610,524) and French patent 2,581,573 (U.S. Pat. No. 5,979,223) filed by the assignee describe various methods and devices intended for continuous measurement of the curve of the resistivity index of a solid sample initially saturated with a first wetting fluid, such as a geologic sample, independently of the capillary pressure curve. The porous solid sample is contained in a sealed sheath placed in an elongate containment cell between two terminal parts. Channels through both terminal parts communicate with an injection system allowing injection of a second, non-wetting fluid into the sample at a first end of the cell and draining of the first fluid out of the cell at the opposite end, through a semipermeable membrane permeable to the first fluid. The sample is contained in a sheath and subjected to a radial pressure by injection of oil under pressure into the annular space between the body of the cell and the sheath. A membrane wettable only by the second fluid is interposed between the sample and the first end of the cell to carry out re-imbibition operations.
Electrodes interposed between the sample and the sample sheath allow application of an electric current and detection of the potential differences that appear between distinct points in response to the application of the electric current. The electrodes are connected to a device measuring the complex impedance of the sample. The longitudinal extension of the electrodes is relatively great in relation to the length of the sample so as to involve the largest possible part of the volume of the sample in the impedance measurements while avoiding short-circuits through the ends of the sample likely to distort the measurements.
One or more injection pressure stages are applied and the continuous variations of the resistivity index as a function of the mean saturation variation are measured without waiting for the capillary equilibria to be established.
Since the annular space between the sheath and the outer wall of the cell are under high pressure, the electric conductors connecting the electrodes to the measuring device run through the outer wall of the cell through sealed ducts (glass bead connectors for example).
In order to improve the measuring accuracy when operating at much higher frequencies in the 100 kHz–10 MHz range for example, it is possible to advantageously use the connection device described in French patent 2,809,821 (U.S. Pat. No. 6,571,606) filed by the assignee, which allows a shielded cable connection of the electrodes to a measuring device, located on either side of a wall separating an enclosure under pressure from the outside medium.